ABSTRACT Clinical thresholds for blood transfusions vary by illness and medical specialty, and are largely based on anecdotal experience. Measured hematocrit is the most frequent determinant for clinical decisions to transfuse blood, often resulting in a one-size-fits-all approach. Although lifesaving at times, blood transfusions have also been clearly shown to increase morbidity and mortality. Thus, an objective measure of cellular oxygenation is needed to work toward a patient-centered approach to blood utilization. A noninvasive and continuous measurement of cellular oxygenation, as proposed in this application, will alert clinicians to the presence and degree of cellular hypoxia and help direct patient-specific clinical decisions. By measuring and analyzing full optical spectra in the visible and near-infrared regions at three source-detector separations, Opticyte's CellSat-100 will quantify myoglobin saturation in muscle cells separately from hemoglobin saturation in the vasculature. Since the goal of red blood cell (RBC) transfusion is to improve cellular oxygenation by increasing blood oxygen carrying capacity, decisions about whether a transfusion is needed would optimally be based on knowledge of pre-transfusion cellular oxygenation. If there is no cellular oxygen deficiency, a transfusion may be unnecessary. In our ongoing Phase I STTR project, we have demonstrated the feasibility of measuring cellular oxygen saturation (ScO2) in healthy adults. Our Phase II activities will center on demonstrating proof-of- concept for accurate, real-time measurement of ScO2 in healthy and critically ill adults. We will build a compact prototype device that can be easily used in a clinical setting. With the new prototype, we will perform a pilot study in an ICU of a major regional referral hospital in Seattle. The Specific Aims of this proposal are to: 1) confirm sufficient spectral quality and stability in our next- generation prototype, the CellSat-100; 2) evaluate the accuracy of real-time ScO2 measurements with the CellSat-100 in healthy adults during ischemia, and; 3) test the hypotheses that ScO2 increases following RBC transfusion, and that initial high ScO2 correlates with better patient outcomes in an ICU patient population. The CellSat-100 will provide unprecedented information about cellular oxygenation that has major implications for both research and clinical care. The long-term goal of the company is to market a noninvasive monitor that will provide robust and accurate measurements of ScO2. The device has broad applicability to critical care, and will be useful in ambulances, emergency departments, intensive care units, and operating rooms. The CellSat-100 will allow patient-specific clinical decisions to be made that prevent prolonged tissue hypoxia leading to improve clinical outcomes.